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add `Vec::extend_from_within` method under `vec_extend_from_within` feature gate Implement <https://github.com/rust-lang/rfcs/pull/2714> ### tl;dr This PR adds a `extend_from_within` method to `Vec` which allows copying elements from a range to the end: ```rust #![feature(vec_extend_from_within)] let mut vec = vec![0, 1, 2, 3, 4]; vec.extend_from_within(2..); assert_eq!(vec, [0, 1, 2, 3, 4, 2, 3, 4]); vec.extend_from_within(..2); assert_eq!(vec, [0, 1, 2, 3, 4, 2, 3, 4, 0, 1]); vec.extend_from_within(4..8); assert_eq!(vec, [0, 1, 2, 3, 4, 2, 3, 4, 0, 1, 4, 2, 3, 4]); ``` ### Implementation notes Originally I've copied `@Shnatsel's` [implementation](https://github.com/WanzenBug/rle-decode-helper/blob/690742a0de158d391b7bde1a0c71cccfdad33ab3/src/lib.rs#L74) with some minor changes to support other ranges: ```rust pub fn append_from_within<R>(&mut self, src: R) where T: Copy, R: RangeBounds<usize>, { let len = self.len(); let Range { start, end } = src.assert_len(len);; let count = end - start; self.reserve(count); unsafe { // This is safe because `reserve()` above succeeded, // so `self.len() + count` did not overflow usize ptr::copy_nonoverlapping( self.get_unchecked(src.start), self.as_mut_ptr().add(len), count, ); self.set_len(len + count); } } ``` But then I've realized that this duplicates most of the code from (private) `Vec::append_elements`, so I've used it instead. Then I've applied `@KodrAus` suggestions from https://github.com/rust-lang/rust/pull/79015#issuecomment-727200852.